Page 272 - CITS - ED - Mechanical

ENGINEERING DRAWING - CITS

The straight line graph
The direct relationship between the two variables is represented by a straight line graph. The formula in the form
of y = mx + c denotes a straight line graph of the relationship between y and x. ‘m’ is the slope of the straight line
with the horizontal and ‘c’ is a constant. If the straight line passes through the origin ‘c’ will be equal to zero. (Fig 4)

Fig 4

Example
To draw a graph illustrating the relationship between inches and millimeters.

X inch 1 2 3 4 5 6

Y millimeter 25.4 50.8 76.2 101.6 127 152.4

The scale chosen for the representation of the points by plotting and joining of the points to get the straight line
graph is shown in the graphics.
In some graphs the points located are to be joined by a smooth curve.
Example

The indicated horsepower of a marine diesel engine for different speeds of the vessel is given in the table below.
To draw a graph to show the relationship and to determine the horsepower when the speed is 9 knots. (Fig 5)

X Speed 7.5 8.8 9.5 10.2 11.3
in
knots
Y i.h.p 750 1100 1350 2000 2400

In this example the ‘x’ axis scale can be commenced from 7.5 itself and the ‘y’ axis from 700, so that the origin
will be 7.5, 700 instead of 0, 0. After plotting the graph to find the horsepower when the speed is 9 knots, draw a
vertical line from the point on the ‘x’ axis at 9 knots to intersect the plotted curve. Draw a horizontal line from this
intersecting point to meet the ‘y’ axis. Read the value and that is the answer. It is 1160.
3x - y = 7
4x - y = 10
To find the values of ‘x’ and ‘y’ graphically.
Equation 1

X 3 4 5 6 7 8
Y 2 5 8 11 14 17

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CITS : Engineering Drawing (Mechanical) - Exercise 11